Human Evolutionary Biology
Honnones do not fossilize. Yet, arguably, they are as important to understanding the evolution of Homo sapiens and other primates as any fossil specimen. The role of hormones in understanding human life history evolution emerges from how genes translate into phenotypes with considerable input from environ mental cues. Most hormones are evolutionarily quite conservative, with very similar if not identical chemical structures between species. Many hormones that flow through the veins of humans
... veins of humans are identical to those that flow through the most exotic vertebrate. Other hor mones and receptors, however, can differ in subtle but important ways between species and even individuals. Honnonal variation, as reflected by circulating levels as well as chemical structure, are of central importance to the evolution of human life histories, both from a macroand microevolutionary perspective. The evolutionary significance of hormones is clearly evident in the multitude of functions that are served, including growth, reproduction, metabolism, and senescence, all of which are central to the evolu tion of human life histories. Hormones are inextric ably involved in the optimal allocation of time and energy. Insulin, leptin, and cortisol, for example, initi ate and manage the flow and assessment of energetic assets such as glucose and fat. Indeed, hormones are involved in life history trade-offs that influence many aspects of human health (Bribiescas and Ellison, 2008) . Testosterone, estradiol, and oxytocin affect behavioral patterns that result in differences in how individuals allocate their time, such as in the trade-off between mate seeking and parenting. In essence, hor mones are a common biological currency that humans and other primates share with other organisms. This allows biological anthropologists to assess the evolu tion of life history patterns in reference to a common physiological aspect, endocrinology (Bribiescas and Ellison, 2008) . Also important to the evolution of life histories is the contribution of hormones to the onset and timing of key life history events (Finch and Rose, 1995) . Childhood growth, reproductive maturation, and reproductive senescence all result from changes in hor mone production. The significance of some changes, such as the decline in estrogens during menopause, remains to be fully understood from an adaptive per spective; however, the impact of these changes on reproductive investment is unequivocal. In this chap ter, we present an overview of how hormones contrib ute to important life history trade-offs, events, and characteristics in humans. In doing so, we introduce and describe various hormones that are illustrative of human life history evolution. The hormones discussed are not meant to represent an exhaustive list. Only a few representative hormones are discussed to illustrate the evolutionary significance of endocrine function in human life histories. HOW AND WHAT IS MEASURED MAKES A DIFFERENCE The amount of hormone that is produced is the most common mode of assessment in contemporary clinical and biological studies, and for good reason. Hormone levels provide useful insights into the physiology of an organism, such as the presence of illness. For example, Graves's disease is the overproduction of thyroid hor mone, resulting in greater than expected metabolic rates and unpleasant symptoms such as bulging eyes. A low level of insulin is indicative of type II diabetes. Yet, absolute levels only provide a partial picture. Variation in hormone levels can result from three basic sources; production, clearance, and bioavailability, usually resulting from carrier protein binding. Produc tion is the amount of hormone that is manufactured and secreted into the circulatory system. This is the most common form of variation. However, hormone levels can also be affected by clearance rates, or how fast the hormone is flushed from the body by the liver and kidneys. Finally, bioavailability, or whether the hormone is capable of activating receptors, can be Human Evolutionary Biology, ed. Michael P. Muehlenbein.